It is known to modify polymers for tailoring their properties. Crosslinking of polymers is one well known modification method in many end applications of polymers. Crosslinking of polymers, such as polyolefines, substantially contributes i.a. to heat and deformation resistance, creep properties, mechanical strength, as well as to chemical and abrasion resistance of a polymer. In wire and cable applications crosslinked polymers, such as crosslinked polyethylenes, are commonly used as a layer material, e.g. in insulating, semi-conducting and/or jacketing layers.
In wire and cable applications a typical cable comprises at least one conductor surrounded by one or more layers of polymeric materials. In power cables, including medium voltage (MV), high voltage (HV) and extra high voltage (EHV), said conductor is surrounded by several layers including an inner semiconductive layer, an insulation layer and an outer semiconductive layer, in that order. The cables are commonly produced by extruding the layers on a conductor. One or more of said layers are then typically crosslinked to achieve the desired properties to the end product cable typically the inner semiconductive and/or outer semiconductive layer.
A well known crosslinking method is crosslinking functional groups, e.g. by hydrolysing hydrolysable silane groups, which are linked to polymer, and subsequently condensing the formed silanol groups using a silanol condensation catalyst, for instance carboxylates of metals, such as tin, zinc, iron, lead and cobalt; organic bases; inorganic acids; and organic acids. The crosslinking of polymers via silane groups thereof is known as silane-crosslinking technology, and for hydrolysable silane groups also called as moisture curing technology. Silane groups can be introduced into the polymer structure 1) by copolymerisation of monomers, such as olefin monomers, with silane-moiety bearing comonomers, or 2) by grafting crosslinkable silane-moieties bearing compounds, such as unsaturated silane compounds with hydrolysable silane group(s), onto a polymer. Grafting is usually performed by radical reaction using free radical generating agents. Free radical generation using free radical generating agents is thus conventionally used e.g. (a) for crosslinking a polymer, i.a. for forming primarily interpolymer crosslinks (bridges) by radical reaction, (b) for grafting a polymer, i.e. for introducing compounds, such as said silane compounds, to a polymer chain (to backbone and/or side chains) by radical reaction, and also (c) for visbreaking a polymer, e.g. for modifying the rheological properties, such as melt flow rate (MFR), by radical reaction. When grafting silane groups containing compounds to polyethylene polymer using free radical generating agents, then also undesirable crosslinking thus occurs as an undesired side-reaction. Crosslinking increases the viscosity of the polyethylene and as a result also the MFR decreases. Highly viscous polymer is difficult to process, e.g. extrude, since high energy input is required in order to achieve sufficient mixing, i.e. homogeneity, and sufficient through-put (production rate) during the processing step. Higher energy-input and thus heat formed due to viscous material naturally can cause undesired degradation of the polymer. The crosslinking side-reaction brings therefore limitation to the amount of silane groups to be grafted, since the more silane groups is added the more free radical generating agent is needed, whereby also more crosslinking side-reactions take place resulting in increased viscosity (decreased MFR) of the polymer. Accordingly, in order to enable the sufficient processability the amount of crosslinkable silane groups and thus the resulting degree of crosslinking of the silane-grafted polyethylene must usually be kept relatively low. Said crosslinking degree can be expressed i.a. as gel content or by measuring hot set properties of the crosslinked polymer material. Thus in the prior art in order to maintain a sufficient processability, said degree of crosslinking of silane-grafted and silane-crosslinked polymers has conventionally been kept at a level which, when defined being the gel content of a crosslinked polymer, corresponds to a gel content of 25-30 wt %, when measured according to ASTM D2765-95 using a crosslinked polymer sample. E.g. semiconductive cable layer materials have typically a high filler, usually carbon black, content in order to provide the desired conductivity property, whereby said filler also increases the viscosity of the polymer material.
Furthermore, it has been found that certain type of carbon black may cause undesired premature crosslinking, known as scorch, which may occur during the production of cable layers resulting in lumps on cables due to too early and uneven gel formation. Such scorch may be probably due to surface properties of said carbon black. Thus in practice, the use of crosslinkable silane-grafted polymers of prior art has been limited, if used at all, in applications, such as in crosslinkable semiconductive layer materials. Accordingly, there is a continuous need for alternative polymer compositions suitable for different application areas.
Thus there is still a need for a simple process to prepare silane crosslinkable polymer composition with improved processability and flexibility which still meet tensile, elongation and cure state target specifications for semiconducting layers in power cables.
EP2164900 from Dow disclose a blend with at least 60 wt % of a silane containing polymer there the blend is diluted with a second component there the hot set decrease with increased amount of a plastomer component. Other patents teach addition of a thermoplastic polyolefin component, for example EP449939, EP736065 & EP1916672. None of them show that a sufficient crosslinking can be achieved.